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Thursday, December 19, 2013

NASA’s planetary science program depends on
regular missions to solar system bodies to gather data. A combination of budget cuts and previous
commitments to develop missions currently in the pipeline means that
development of follow on missions may slow to a crawl. In this post I look at the current situation
and NASA’s plans and then look at options the agency may consider if budgets
remain tight into the next decade.

Space News recently
carried an article
that described how the mission roadmap laid out in the recently completed
Decadal Survey was no longer affordable given cuts to NASA’s budgets. A single large mission threatens to crowd out
the start of smaller missions that would enable regular flights. Space News quotes a leading scientist in the
field who says, “The
result is ‘a strategic program that is drawn out in a way that is acceptable to
no one’”.

The roadmap in question is for NASA’s
heliophysics program, which has a budget about half the size of the planetary
science program. The large mission is
the Flagship Solar Probe Plus spacecraft that will make multiple close passes to the sun
and is expect to have a cost of $1.2B to $1.4B.

However, by changing a few names and details,
the article could have been written for NASA’s planetary, astronomy and
astrophysics, or Earth science programs.
Whether you agree with the politics or not, the budgets for many federal
discretionary programs are shrinking.
NASA’s budget has not been spared, and the science program budgets are
feeling the effects. (For a good summary
of the planetary program’s budget woes, see these posts by Casey Drier on the
Planetary Society’s blog: historical
trends and current
issues.)

Earlier this decade, NASA completed roadmap
plans (called Decadal Surveys) for each of its science programs (except the
Earth science program, which completed its Survey in 2007). All the Surveys were told to base their plans
on projected budgets that turned out to be wildly optimistic. As a result, the plans included a mixture of
large and smaller missions that would ensure frequent flights to explore
multiple targets. For the Planetary
Science division, the authors of the Decadal Survey recommended for the coming
decade development of one to two Flagship missions (in the $1.5B to $2.5B
range), two new Frontiers missions (~$1B), and several Discovery missions (~$500M).

If the Planetary Science Division’s budgets had
remained at the levels projected when the Survey was drafted, we would be looking
forward to an incredibly rich program of missions to many solar system
destinations.

Now following the budget cuts, projected budget
shows that the Planetary Science Division will struggle to keep missions in
development in all three of its size classes.
Instead of starting development for six or more new flights to the
planets, we will see just three new missions launch through the end of the
decade and perhaps only one in the first part of the next decade.

Approved missions in
development (solid lines) as well as projected funding for future missions
(dashed lines).

Through 2015, the planetary program will
simultaneously develop two smaller missions, the New Frontiers OSIRIS-REx that
will return a sample from a primitive asteroid and the Discovery InSight
mission that will study the interior of Mars

For the Planetary Science Division, the squeeze
will begin in 2016 when it begins to heavily spend on the development of the
$1.5B 2020 Mars rover. To afford this
large mission, the budget plans show cuts in budgets elsewhere. In 2015, the budget for outer planet missions
that funds the continued operation of the Cassini orbiter at Saturn is
projected to substantially shrink. (See my
post here on the potential early termination of the
Cassini mission.) At the same time, funding
to develop future New Frontiers and Discovery missions drops dramatically.

The problem NASA faces is compounded by the
funding pattern missions in development follow.
While missions typically take four to five years to move from approval
to launch, most the funding comes in the middle of years of development in what
is known as peak funding. To develop
missions that will launch in the early part of the 2020s, NASA needs to be able
to meet a peak funding level somewhat greater than $200M at the same time as
the 2020 rover hits its (much higher) peak funding needs.

Based on the projected budgets, it appears from
NASA’s budget documents that there just won’t be enough money to sustain
mission in development in all three of its mission classes at the end of the
decade. However, if NASA were to combine
projected funding for its New Frontiers and Discovery programs at the end of
the decade, it appears that it could support development of one mission along
with the 2020 rover.

Based on statements from the Planetary Science
Division’s chief, Jim Green, at the just completed American Geophysical Union
conference, that appears to be what NASA plans to do. NASA will request proposals for the next
(fourth in the series) New Frontiers mission around 2015. NASA would then likely select the winning
proposal in late 2016 or 2017. Given
approximately five years for development, the new mission likely would launch
in 2021 or 2022.

In case the budget situation improves, NASA
plans to also complete the preparatory work to request proposals for a
Discovery program in the same time frame.

However, the plan for developing a New
Frontiers mission at the end of the decade depends on the 2020 rover being
developed at a cost near $1.5B. That
mission currently is in the early formulation stage and a final budget hasn’t
been set. If the rover costs come in
substantially higher, then affording simultaneous development of a New Frontiers
mission could be difficult at best and or impossible.

It takes so long to plan and develop planetary
missions that NASA has limited options for restructuring its program of
missions to deal with tightening budgets through the end of this decade. But if fewer dollars to develop missions is
the long term story, what other options could NASA explore?

Everyone agrees that having NASA’s planetary science
program consumed by a single large mission creates too much risk (what if the
mission is lost at launch?). Because a
single mission will necessarily explore just a single destination with a small
suite of instruments, most of the scientists in the field will be starved for new
data. (The 2020 rover, for example, will
not explore what drives global climate patterns, explore whether Venus was once
habitable, or explore how the planets formed.)
NASA also needs to have a cadre of scientists and engineers who have
participated in developing missions to become the leaders of future missions.
That’s difficult to do with a single mission in development.

One idea that has been discussed within NASA
has been to reduce the number of Flagship missions to enable funding more small
missions. There’s also been a history of
Flagship missions regularly experiencing severe cost overruns, starving the
rest of the science program. NASA’s
administrator, Charles Bolden, recently told a science advisory panel that NASA
can’t afford Flagship missions any more.
NASA later explained that NASA was not abandoning Flagship mission and
understands that some key science questions can only be tackled with these
large missions.

My take is that NASA wants to limit its
exposure to Flagship missions for the reasons given above. At the same advisory panel meeting, the
Associate Administrator for Science, John Grunsfeld, said he wanted to begin
studies that would look at whether substantial portions of high priority Flagship
missions could be done as New Frontiers missions (perhaps with a bump of 25% or
50% in the cost cap).

During the Decadal Survey process, a number of
mission concepts were studied, (including all those that made the candidate
list for New Frontiers missions). No
studies were done for New Frontiers-class missions to either Europa or Titan
because these destinations were studied as Flagship-class missions. (An analysis of missions for Enceladus
suggested that a multi-flyby mission to explore its plumes might fit within a
New Frontiers or a New Frontiers-plus budget although the full cost analysis
was done for a more expensive Enceladus orbiter.)

Could a New Frontiers mission be done for
Europa? The Decadal Survey analysis
concluded that a multi-flyby mission for Jupiter’s moon Io could be flown as a
New Frontiers mission. An Io mission would have to deal with the same key
issues a Europa mission would face – requirements to host both remote sensing
and in-situ instruments, the high radiation fields of the inner Jovian system,
generating power far from the sun, and communicating large amounts of data back
to Earth. The Io Observer study results
suggest that a New Frontiers mission for Europa is a credible concept to study. However, compared to the proposed ~$2B Europa
Clipper multi-flyby Flagship mission, many key questions about Europa couldn’t
be addressed by a single New Frontiers mission.

I compared the instrument list of the Io
Observed with the proposed instrument list of the Europa Clipper. If a Europa New Frontiers carried a similar
instruments as the Io mission, it might address the key questions for geology
(a moderate resolution camera), surface composition (an imaging spectrometer
and neutral mass spectrometer), and partially explore questions about the icy
shell and ocean (gravity science, magnetometer, plasma spectrometer). However, this would drop the power- and data-hungry
ice penetrating radar and high resolution landing site reconnaissance
instruments. A New Frontiers mission
also might make many fewer flybys than the 30+ planned for the Europa Clipper
(the Io Observer concept assumed just six to ten flybys) and might return much
less data per flyby. (Note: A mission
definition team would likely come up with a different list of instruments than
I did in this simple thought experiment.)

Another possibility for NASA would be to
include even cheaper missions than Discovery missions in its program mix. A committee of scientists reviewing options
for the heliophysics program recommended that NASA develop technology to enable
scientific missions with very small spacecraft, SmallSats
and CubeSats. I have previously written about the potential
and limitations for these small missions that could cost just a few $10Ms for
SmallSats and $1Ms for CubeSats. Because
of the small size of these spacecraft, the science they could do would be
limited. However, several could be flown
in a decade increasing the variety of science conducted. Perhaps more importantly, these missions
could be used to train a new generation of scientists and engineers to become
leaders on larger missions.

Through the end of the decade, NASA’s planetary
managers have limited options. They
can’t, for example, just dictate a Europa New Frontiers mission without first
doing an intense analysis of the concept.
They can’t simply decide to introduce SmallSat missions before they know
the designs are feasible, the technology is ready, and that there would be
scientific value for the money spent.
For mission development in the next decade, NASA has time to fully
evaluate these options.

If NASA has a similar budget for new mission
development in the 2020s as it is projected to have the rest of the decade,
then it would have an inflation-adjusted $5B to $6B over the decade. With that funding, it could, for example, fly
two to three New Frontiers or New Frontiers-plus missions and approximately
four Discovery missions and perhaps several SmallSat missions. It would not be as rich a program as we had
in the plusher years of 2000 to 2012. It
could be a richer program than we seem likely to develop this decade.

Also, budgets are not fixed. While we have seen that they can go down,
they can also go up. Even modest
increases of $100M per year over the current projections of ~$1.2B per year would
go a long way towards enabling a richer program this decade. If you haven’t participated in the Planetary
Society’s campaign to lobby for increased funding, I encourage you do so.

Appendix: Approved list of New Frontiers
missions

One of the tasks for the Decadal Survey was to
develop a list of missions that scientists could propose for competitions to
select New Frontiers missions. The
Survey approved five missions for the next selection (New Frontiers 4) and
approved an additional two for the selection after that (New Frontiers 5). The latter are indicated with an
asterisk. NASA may consider expanding
this list to partially fulfill the goals the Survey identified for recommended
Flagship missions such as a mission to Europa (see above).

Thursday, December 12, 2013

I wrote a post on possible low cost missions to Enceladus and Titan a few weeks ago. Then NASA
announced that it was cancelling the ASRG power systems that the two
mission concepts described depended upon.
There no longer seemed a reason to give an update on missions designed
to explore the active plumes of the icy moon Enceladus at Saturn.

The location of water vapor detected over Europa's south pole in observations taken by NASA's Hubble Space Telescope. Credit: NASA/ESA/L. Roth/SWRI/University of Cologne

If follow up observations show the plumes of Europa to be persistent,
then I would be shocked if no one proposed a Discovery-class ($425-500M) to
explore those plumes up close. To play
armchair mission architect (and remember that I am a rank amateur), one could
imagine a small spacecraft with cameras or a UV spectrometer to observe and characterize
the plumes from afar (say from the relatively low radiation environment near
Ganymede’s orbit). Then the spacecraft
might dip down one or more times to Europa’s orbit to fly through the plumes
and sample its composition with a mass spectrometer. During the flybys, a thermal imager or a
mapping near infrared spectrometer might study the surface surrounding the
sources of the vents as well as map other regions of Europa. (However, the $2.1B proposed Europa Clipper
with its many instruments and radiation hardening for many flybys would be far better if budgets allow.) Or a mission might capture ice
particles and return them to Earth. As a
bonus, spacecraft studying Europa could be solar powered, preserving our supply
of Plutonium-238 for other missions.

I’m not a mission architect, but the mission concepts described below that have been planned by experts to explore Titan and Enceladus suggest what a
highly focused mission might do for an icy world with plumes. And I’ve just learned from a presentation
made at the Venus Analysis Group last month meeting that NASA appears to be
considering making MMRTG Pu-238 power systems available for the next Discovery
mission. That likely would be an
enabling decision for the concepts described below for the moons of Saturn
where solar power would be an iffy proposition for a tightly cost constrained
mission.

My original write up of the proposed Discovery-class missions to
Enceladus and Titan follows.

------------------------

Over the years of following planetary mission proposals, I’ve come to
be especially intrigued by a few of them.
Last summer at the Low Cost Planetary Mission conference, new details
were presented on two of my favorites, which would explore Titan and Enceladus.

Discovery missions are NASA’s smallest planetary missions that in the
past have been capped at a cost of around $425-500M for the spacecraft,
instruments, and operations (launch costs are counted separately). So far, no Discovery missions have traveled
past the asteroid belt (where the Discovery Dawn mission is operating). Fitting a mission to the outer planets into
the Discovery budget has proved challenging.
To operate far from the sun, these missions usually require
plutonium-based power supplies rather than solar panels. NASA has offered to pick up a substantial
portion of the costs of adding these power systems to a Discovery
spacecraft. However, proposers
have commented that there are still substantial ancillary costs to their
use that effectively imposes a penalty to missions that require them. In addition, flights to the outer planets
take longer to reach their destination, and each year of additional flight
costs $7M to $10M from a budget that is already tight.

These challenges haven’t kept scientists from proposing exciting outer
planet missions (see these posts on a Uranus
concept and an Io
Concept). The scientific community also
has proposed changes to the way NASA structures the Discovery program,
including raising the overall cost cap to $500M and adjusting the budget for
the length of the time needed to reach a destination. If these proposals are adopted, then
Discovery missions to Enceladus and Titan would be easier to do.

Of the two proposals discussed last summer, the proposal to return
samples from the plumes of Enceladus is the more audacious. The mission would build upon the sample
collection methods used by the highly successful Stardust mission that returned
dust collected during a high speed flyby of a comet. (Several of the mission’s proposers were
members of the Stardust team.) Like
Stardust, the Enceladus mission would use an ultra-low density material called
aerogel to capture dust particles embedded within the ice particles spewed into
space by Enceladus. A second sampling
system that would capture the volatile icy materials may also be used. (A similar concept was dropped from Stardust
to minimize costs but will be flown on the Hayabusa-2 asteroid sample return
mission.)

The jets of Enceladus. Credit: NASA/JPL/Space Science Institute

The concept presented at the conference was an evolution of a previous
version of this concept that I covered in the past. The major difference is that the new concept
has dropped instruments for imaging the sources of Enceladus’ jets and
measuring composition of the jets in-situ.
(Although the design remains compatible with the latter if funding were
available.)

Sample return missions like this proposal were considered by the last
Decadal Survey, and two key challenges were identified. The first was to ensure that the Earth could
not be contaminated with any life brought back from Enceladus. The Survey report assumed that an expensive
sample containment facility would have to be built. The current mission concept proposes to
address this problem by tuning “the sample impact velocity such that large
molecules would survive, but structures such as bacteria or viruses would not.” However, NASA still could decide that any
sample return from a potential life-bearing world requires expensive new
facilities.

The second challenge for the sample return mission would be that the
mission incurs the costs of flying both to the Saturn system and returning from
it, resulting in a 15 year flight (4.5 years of that spent returning to Earth).

My hope is that the proposers can satisfy NASA on these
challenges. As the presentation states,
“Determining the presence or absence of life requires multiple analyses that
are too complex to fly, and the optimal analytical sequence to follow depends
on what you learn at each step of the sequence... Regardless of any biological
content of the sample, this would be the first sample from the Outer Solar
System and would provide invaluable clues to the origin and evolution of the
Solar System.” Or put another way, the
ultimate goal for every class of worlds is to return samples back to Earth
where the exquisitely sensitive instruments in laboratories can address the
most subtle questions. This mission
proposes to do this in a way that is simple, low cost, and uses the experience
of a team that flew a similar mission.

The second mission presented last summer was the Journey to Enceladus
and Titan (JET) mission that was the subject of a previous
post (that post has considerably more detail on the science goals than this
post). JET would build directly on the
discoveries of the Cassini orbiter at Saturn.
While Titan’s surface is perpetually hidden by haze across most of the
spectrum, Cassini discovered infrared windows that allow direct imaging of the
surface. Unfortunately the Cassini
instrument that can see in those spectra produces low resolution images (~3 km
during close flybys). Cassini also has
a radar imager, but its best resolution is in the hundreds of meters. The result is that our current maps of the
surface of Titan have a mixture of resolutions and colors similar to what we
had at the end of the 1960s following the Mariner flyby missions.

The highest resolution in this composite
image of Titan’s northern lakes region taken by Cassini is 3 km. Credit: NASA/JPL-Caltech/University of
Arizona/University of Idaho

Imaging the sources of Enceladus’ jets also is best done in the
infrared where the heat of the material escaping into space makes the vents
glow in comparison to the frigid surrounding surface.

Cassini thermal image showing temperature
differences in the areas from which Enceladus’ jets orginate.The hot spots are shown on top of much higher
resolution visible light images.Credit:
NASA/JPL/GSFC/SwRI/SSI

The JET mission would carry a high resolution thermal imager to map the
surface of both moons. At Titan,
substantial portions of the moon would be mapped in four thermal bands
(“colors”) at resolutions of 25 to 100 m with other areas imaged at coarser
resolutions (but still better than Cassini).
Coverage would include high priority areas such as the Huygens probe
landing sites, the lakes in the northern polar region, and the hilly and
mountainous Xanadu region. The resolution
images of the sources of Enceladus’ plumes will be higher still since the flyby
altitudes will up to ten times closer (although the relative speed will also be
higher causing smearing).

JET’s second instrument, a duplicate of the Rosetta comet mission’s
mass spectrometer, also builds on Cassini’s results. When Cassini was planned, scientists expected
that the gasses they could sample at the top of Titan’s atmosphere would be
simple as would be the dispersed ice particles between the moons. Now we know that the top-of-atmosphere
chemistry is more complex and that Enceladus is spewing gases and ice particles
containing complex salts and possibly complex organic compounds into
space. JET’s mass spectrometer would be
able to measure the composition of material with ten times the atomic mass of
Cassini’s instrument, with 100 times better resolution of compounds, and a
thousand times the sensitivity.

As presented at the conference, the JET proposal hasn’t changed since
it was first discussed publicly several years ago. However, new information on the coverage
possible at Titan and the design of the spacecraft were included.

Current design for the JET
spacecraft. Credit: JPL

My understanding is that both these concepts will be proposed for the
next Discovery competition (date uncertain, depending on future NASA
funding). Like the Uranus Discovery
concept I previously wrote about, these are examples of the tightly focused
outer planet missions scientists and engineers think could be flown in this
program. The keys to fitting under the
cost caps are minimal operations during the long flights to their destinations
(spacecraft hibernation), small instrument compliments (one each for the
Enceladus sample return and Uranus flyby, two for JET), and simple mission
operations once at their target worlds.
If these measures are sufficient, then we may see the outer solar system
opened up to a new class of missions.

Sunday, December 8, 2013

The managers of NASA’s planetary exploration are caught between that
proverbial rock and a hard place. Given
their situation, I admire them for making a tough call to cancel an advanced
technology program that could have enabled a new generation of small missions
to several exciting destinations in the solar system.
The good news is that in the end, the cancellation may make little
difference in the missions that actually will fly. That’s the bad news, too.

The cancellation was for NASA Advanced Stirling Radioisotope Generator (ASRG)
that would have provided a new generation of plutonium-238 (Pu-238) based electrical
power systems. ASRG’s could have stretched
NASA’s limited supply of plutonium to potentially enable missions to the perpetually-shadowed
polar craters on our moon, to flyby Uranus, or to float for months on a Titan
lake.

NASA has an alternative, flight-proven generator technology, the
Multi-Mission Radioisotope Thermal Generators (MMRTGs). Because MMRTGs use approximately four times
the Pu-238 as ASRGs for a similar power output, NASA now will need to hoard its
Pu-238 supply to power its largest expected missions of the 2020s. That would enable the planned 2020 Mars rover
and the hoped for Europa Clipper multi-flyby missions. Supplies for follow-on missions would not be
available until late in the 2020s for a small mission and the early 2030s for a
larger mission.

MMRTG

ASRG

Power

110 We

140 We

Mass

45 kg

~20 kg

Efficiency

6-7%

~30%

Pu-238 Fuel

4.8 kg

0.8 kg

Comparison of the MMRTG and ASRG power
systems.

In my post today, I want to give the background for NASA’s decision
(that rock and a hard place) and to talk about the types of missions that can
still be flown and those that would be lost.

The “rock” is NASA’s present tight budget for planetary exploration,
which has been cut in each of the last several years. NASA recently learned that bringing ASRGs to
flight ready status would cost $100M more than originally expected. By cancelling the ASRG program, NASA saves
$170M over the next three years.

NASA has not publicly identified where the savings would go. They could use the funds for other NASA
programs or Congress could lower NASA’s budget by that amount. However, if the planetary program is allowed
to keep the savings, $170M could go a long way to stretching thinned budgets
for missions or research. (Coincidentally,
$170M would be almost enough for the Cassini spacecraft, which may otherwise be
turned off in 2015, to fly until its fuel is exhausted in 2017.)

The “hard place” is NASA projected budget for planetary
exploration. While Congress appropriates
funds for each current year, the President’s Office of Budget and Management provides
estimates for future budgets. NASA’s
managers can only start a new mission when those projected budgets show room in
the budget for a new mission. Right now,
NASA’s planetary science projected budget shows no room to start a new mission
that would launch before the end of the decade and more likely in the early
2020s. At the rate of mission flights
suggested by the projections, there would be little demand for
plutonium-powered missions beyond the current Pu-238 supply.

Examples of funding peaks for NASA’s current approved smaller planetary missions along
with projected funding for the next generation of missions. OSIRIS-REx is a New Frontiers program mission
and the Mars MAVEN and InSight missions are Discovery program-class missions.

NASA’s missions in development follow a roller coaster funding profile
with development requiring high peak funding for a year or two. As one mission rolls off its peak funding,
funds become available to start the next mission. Projected funding (see chart above) suggests
that budgets won't support the funding of a new mission until the end of this decade, with a launch then or in the early 2020s. As a result, the new missions that might
require plutonium power supplies are projected to be – unfortunately – too few
and far between.(Part of NASA’s future
mission challenge is that late in this decade most of its new mission budget
will go to the $1.5B to 1.7B 2020 Mars rover.)

The U.S. has approximately 30 kilograms of Pu-238, or enough for five
MMRTG’s. A recently released
presentation slide from NASA shows one of those MMRTG’s reserved for the 2020
Mars rover, which will be a close copy of the MMRTG-powered Curiosity rover now
on Mars. The other four are held in
reserve for a mid-2020’s mission or missions.
While not stated in the presentation, this could be the proposed Europa
Clipper multi-flyby mission that would require the equivalent of four MMRTG’s
electrical power.

NASA’s expected supply and use of
plutonium-238 for the next two decades.
Credit NASA (see this presentation).

The U.S. has just approved plans to produce new Pu-238 for the first
time in decades. The amounts will be
small, around one kilogram a year. At that
rate, approximately two MMRTG’s could be fueled in a decade with new
Pu-238. However, the U.S. has a
stockpile of degraded Pu-238 (that presumably consists of material old enough
that a significant proportion has become useless because of radioactive decay). NASA plans to mix its new Pu-238 with
reworked older material to produce enough useable material to power several
MMRTG’s in the 2020s.

Given NASA’s new dependence on MMRTG’s, what types of missions can it
still fly and which become impossible or unlikely?

Answering that question requires understanding when a radioisotope
power supply either is absolutely necessary to fly a mission or would
substantially enhance it. The
alternative is solar power from solar photovoltaic panels. Too far from the sun, and sunlight is too
feeble to power a spacecraft. Until a
few years ago, that line of demarcation fell somewhere in the asteroid
belt. With improvements in low light and
low temperature (it’s cold far from the sun) solar cells, solar powered
missions at Jupiter are feasible. One (NASA’s
Juno) has launched, a second (Europe’s JUICE mission) will launch in 2022, and
the proposed Europa Clipper mission could use solar power.

Several studies have looked at using solar power for missions to
Saturn. The low light, low temperature
solar cells should work there. The major
problem is that at Saturn, a set of solar panels like those on the Juno
spacecraft that produce ~440W at Jupiter would provide only ~110W at
Saturn. For the Juno spacecraft, approximately
half of its 440 watts of power will go to powering the spacecraft systems and instruments
and half will go to running heaters to keep the spacecraft warm. Keeping warm is even a bigger problem at the
more distant Saturn, and after running heaters, little electrical power might
be available for anything else. More
solar panels could be added at the cost of additional weight and fuel to
maneuver with that weight. As the
appendix at the end of this post states, solar power at Saturn is technically
possible, but the trade offs are significant.

Beyond Saturn, the size and weight of solar panels would become
prohibitive. Realistic spacecraft for Uranus,
Neptune, and destinations beyond require radioisotope power supplies (or a
future generation of solar panels).

Other missions requiring Pu-238 missions are those where solar power is
intermittent or unavailable.This would
include long-term landers or rovers for the perpetually shadowed craters at the
poles of the moon or the surface of Titan.Long-lived lunar stations would benefit from radioisotope power since
they must survive nights lasting 14 terrestrial days.Mars rovers also benefit from Pu-238 because bright sunlight is available only for part of the day, winter brings dimmed solar
light, and dust storms can make noon as dark as twilight.Another challenge for these types of missions
(and those in the outer solar system) is keeping the spacecraft warm.Pu-238 power systems have lots of excess heat
that can be used without diverting electrical power to heaters.

Comet hopper mission concept. The ASRG unit is the rectangular box with cooling fins on top of the spacecraft. Credit: NASA/GSFC/University of Maryland

Some inner solar system missions can’t be done with large solar panels. One clever idea was a comet hopper (called
CHOPPER) that would land in multiple places on the surface of a comet. Landing repeatedly on a rough surface with large solar
panels would be impractical, making a Pu-238 power system an enabling
technology.

The AVIATR airplane concept at Titan depended on the low mass of the
ASRGs to keep it light enough to fly.
With only MMRTGs available, that concept is unfeasible. However, hot air balloons for Titan couldn't stay aloft with the lower waste heat available from ASRGs to heat the air for their balloons. This type of mission
requires the heat from the additional Pu-238 in an MMRTG.

NASA has a list of missions it would like to fly in its Flagship ($1.5B
to $2B) and New Frontiers ($750M to $1B) programs. The following table summarizes whether solar
power could be used, and if MMRTG’s would be a benefit, how many would be
useful. These data are from mission
studies done for the last Decadal Survey effort to plan NASA’s planetary
science program. The studies assumed
ASRG’s, but MMRTG’s provide similar levels of power, so if a mission requires
two ASRG’s, it is likely two MMRTG’s would be required. (Many of the studies looked at multiple
configurations with different numbers of ASRG’s, and I’ve shown the minimum
number for a credible mission. In
addition, ASRG’s, unlike MMRTG’s, have moving parts. For redundancy against mechanical failure,
all studies assumed at least two ASRG’s.
It is possible that some of the missions could be done with a single
MMRTG.)

Flagship

Possible with solar power?

Number of ASRGs/ MMRTGs

Mars 2020 rover

Yes*

1

Europa Clipper

Yes

4

Uranus orbiter

No

3

New Frontiers

Comet sample return

Yes

--

Lunar sample return

Yes

--

Saturn probe

Probably

2

Trojan asteroid orbiter

Yes

2

Io observer

Yes

2

Venus lander

Yes

--

Lunar network

Yes

4

*MMRTG is likely to
minimize design changes to the Curiosity rover design it will be based on

Radioisotope power requirements of NASA's candidate Flagship and New Frontiers planetary missions. From the Decadal Survey mission studies.

Based on this table, a reasonable question might be whether the
cancellation of the ASRG program will impact the missions that fly. From the list of candidates for Flagship and
New Frontiers missions, the answer may be no.
There’s sufficient Pu-238 for the 2020 rover and Europa Clipper (which
could switch to solar power anyway).
Under current budget forecasts, a Uranus orbiter wouldn't launch before
the new production of Pu-238 becomes available.
All the New Frontiers missions on the list could be done with solar power although this would like incur design challenges for the Saturn probe and lunar network.

Where we may see a loss is in the lowest cost class of missions, the
Discovery program ($425M to $500M). The
expectation had been that NASA would make at least one pair of ASRGs available
for a Discovery mission. Engineers and
scientists came up with clever ideas for ASRG-based missions – the comet
hopper, a Titan lake lander, an orbiter to revisit Titan and Enceladus, a Uranus
flyby, and others. With MMRTGs now the
only option, NASA needs to hoard its supply of Pu-238. It also has lost the motivation to test a new
technology – ASRGs – on a relatively low cost mission. MMRTG technology is already proven.

So NASA’s managers made the tough call, and if I were in their shoes,
I’d have done the same. I do see two glimmers
of hope to resurrect those clever Discovery missions though, or to give
engineers the flexibility to use MMRTGs for New Frontiers missions. The first is that Congress for the 2013
budget made it clear that it wanted higher funding for future missions. If this desire becomes policy (and the
President’s budget office would have to reflect this in their budget
projections), then there may be more demand for Pu-238 than NASA is currently
envisioning. The second is that if the
Europa Clipper design team decides to go with solar power instead of MMRTGs,
NASA will have more Pu-238 than missions planned to use it. In this case, they might make MMRTGs
available for New Frontiers or even Discovery missions.

As part of the 2012 Decadal Survey to create a roadmap for NASA’s
planetary missions, studies of a number of potential missions were
conducted. Two looked at smaller
spacecraft to carry and atmospheric probe to Saturn and relay its data back to
Earth and to conduct multiple flybys of Enceladus. Both studies concluded that solar cells were
possible, but radioisotope power systems were preferable. The following excerpt from the Saturn probe mission study report discusses the challenges of solar power at Saturn.

“Regarding power systems,
although it might be possible to use solar arrays for the carrier-relay
spacecraft’s primary electric power system, operating at 10 AU would push the
very limit of current solar cell technology, requiring large margins and an
expensive parts selection program for the solar cells. For the mission time
period studied, a radioisotope power source (RPS) is less expensive and lower
risk for this mission than a solar array system, and would perform well in all
mission phases.

“Unexpectedly, the solar vs. nuclear trade study concluded that the
nuclear option, specifically the use of ASRGs, would provide significant cost
savings and risk reduction relative to the solar option. There are multiple
reasons. Despite using no RPS, the solar option would nonetheless require
radioisotope heater units (RHUs), some in the proposed probe and some in the
proposed carrier-relay spacecraft. Thus it would incur some costs associated
with nuclear payloads anyway, nullifying one potential cost-saving advantage of
the solar option. Solar cells from a production process are not all exactly the
same, and differences that are small under normal illumination conditions could
be greatly magnified under the low intensity, low-temperature (LILT) conditions
in the outer solar system. For a mission to Saturn, selecting acceptable solar
cells from production batches would require a significant program of testing
and screening, increasing the cost per cell. Because such testing does not
always guarantee expected performance, the solar arrays would need to be
designed with somewhat larger margins, increasing the size, cost and risk of
producing and flying already-large arrays. These large arrays would have masses
far greater than the mass of ASRGs producing similar power. Solar array size
and mass influenced the proposed launch vehicle selection and subsequent
spacecraft operations: The dimensions and mass of the arrays would require a
larger launch vehicle and significant operational constraints, contributing
significantly to the total cost and risk difference.”

Tuesday, November 12, 2013

In my post yesterday on the potential loss of the last years of Cassini's extended mission at Saturn, I quoted the wrong costs for operating Cassini. The correct figure is $60-65M per year, and not the $50-55M. (The latter reflected a potential cut -- at the cost of a reduction in science conducted -- proposed for a past funding review.)The following link provides a lot of detail on the current extended mission (through the end of Fiscal Year 2014) and the science results: http://www.lpi.usra.edu/opag/mar2012/presentations/OPAG_Thurs_morning/5_Cassini_Status_Spilker.pdf

Update, August 2014. NASA has now committed to fully funding the Cassini mission to it's planned end in 2017.NASA’s shrinking budgets for
planetary exploration may force it to decide between continued funding for the
Saturn Cassini mission and the continued funding for its Mars missions. Mars would seem to be the likely winner. If so, we would lose a dramatic ending to the
Cassini mission that would be essentially an entirely new mission to Saturn. The ring and cloud-top grazing final orbits would
produce entirely new science and would be as rich as the Juno mission to
Jupiter.

Note: I've corrected this post to reflect Cassini extended mission costs of $60-65M per year instead of the $50-55M in the original post.

Comparison the possible end-of-mission orbits for Cassini and the orbits for Juno. By dipping close the rings and atmosphere, Cassini can make highly detailed measurements of the rings, atmosphere, and Saturn's interior. The Juno mission en route to Jupiter will provide similar measurements for Jupiter's atmosphere and interior. Source: NASA's Outer Planets Assessment Group (OPAG).

Operating planetary missions is an expensive proposition. This year, NASA will spend something like
$215M or ~18% of is planetary science budget operating its fleet planetary
spacecraft that have already launched.
(These numbers are approximate because NASA doesn’t separate out the
costs of operating the Cassini and Lunar Reconnaissance Orbiter missions.)

The initial costs of operating a mission during its early phases, what is
known as the prime mission, are included in the funding budgeted specifically
for the mission. Once a spacecraft
continues operating past its prime mission (most do), it enters what is known
as an extended mission. NASA budgets
money to operate extended missions. The amount
planned for the next several years is fixed and doesn’t go up with the number
of extended missions. As a result,
there’s a process known as the Senior
Review that judges which missions should continue to be funded and at what
levels to divvy up the pie.

For the past several months, NASA
has been warning the planetary community that there will be insufficient
funds to operate its all extended missions at their current levels.

The Senior Review encourages mission teams to both be clear on what new
science is enabled and to find ways to continue to operate on smaller budgets
than were used for operations in the prime mission. It’s not unusual for the science return in an
extended mission to equal or exceed that of the preceding prime mission.

In general, mission teams use three key justifications for
funding extended missions:

In the prime mission, it’s not unusual for new
discoveries (e.g., the plumes of Enceladus) to be made, but scientists aren’t
sure what they mean or what processes drive them.Those discoveries become the subject of
follow up observations (e.g., multiple passes through the plumes to conduct a
variety of targeted measurements) that greatly deepen our understanding.

Many processes simply require a long period of
time to observe.For example, the Mars
Reconnaissance Obiter has imaged and re-imaged many seemingly blank spots on
the surface of Mars.At some of those
spots, small meteor strikes revealed ice lying just below the surface.The ice later sublimated away; only repeat
imaging revealed the ice during the brief time it was visible.

An extended mission may allow the spacecraft to move
to a new location for entirely new observations.The extended mission of the Opportunity rover
has allowed it to essentially become a “new” mission at least twice by arriving
at locations that were considerably different than those visited in the prime
mission.In another example, the lunar
GRAIL orbiters used their extended mission to map the moon’s gravity field from
a much lower (and riskier) orbit than the orbits of the prime mission,
resulting in higher resolution maps.

The budget problem for NASA is that the demand for extended mission
funding will jump in FY15 when Curiosity begins its extended mission. At best, NASA’s budget for extended missions
will be flat (news accounts quoting NASA managers) or be cut by eliminating
funding for Cassini mission operations.
(The latter is suggested by NASA’s budget documents which show the
budget that funds Cassini dropping to levels that don’t support the extended
mission.)

Demand up, budget flat or down, something may have to go.

The Senior Review will look across all missions and could recommend
funding Cassini instead of one or more Mars missions. However, that seems unlikely to me. The Curiosity mission is just beginning what
is likely to be a decade of exploration.
The Mars Reconnaissance orbiter is required to help select landing sites
and travel routes for the planned 2020 Mars rover as well as relay data from
the rovers. The Mars Odyssey orbiter is an essential data relay for
currently operating rovers. And who
would suggest turning off the venerable Opportunity rover? (Neither of the last options provides much in
the way of savings anyway; they are lean, mean extended missions.)

Cassini has had a long run at Saturn, having entered orbit in 2004 and
been operated as an extended mission after the first four years. Between the end of its current extended
mission in late 2014 and the time the spacecraft is expected to run out of fuel
in 2017, the spacecraft could continue to follow up on past discoveries (for
example, with three final Enceladus flybys).
It also would continue to look for seasonal changes at Titan and
Saturn.

However, in its last year of possible operation before its fuel is
exhausted (November 2016 to September 2017), Cassini could become an entirely
new mission.

Up until November 2016, Cassini will have stayed well away from
Saturn’s rings (except during orbit insertion in 2004) because the risk of
catastrophic collision with an ice particle or boulder would be too high. With the fuel almost exhausted, though, Cassini’s
managers want to bring it in close to the rings and Saturn itself. Twenty orbits would carry the spacecraft just
outside the rings for close up observations of their structure and mass. (The latter measurements, for example, would
help scientists determine how old the rings are.)

Following these orbits, the spacecraft would slip into the gap between
the inner most ring and the top of Saturn’s atmosphere. From these 22 close-in orbits, the Cassini
mission would essentially replicate the measurements that will be made at the
same time by the Juno mission to Jupiter:
detailed measurements of the interior of Saturn and of its
atmosphere. As a bonus, scientists can
make more detailed measurements of the rings.
And for all of us who vicariously explore the solar system through these
missions, think of how beautiful the images would be looking out at the rings
and Saturn from just above the clouds.

At the end of the mission, its fuel gone, Cassini would enter and burn
up in Saturn’s atmosphere, an end that would prevent it from accidently
impacting and contaminating Enceladus or Titan with micro-organisms from Earth.

All of the science in this end-of-mission scheme would be new, never done
before science because extending the mission would allow Cassini to go to
somewhere new. NASA thought that these measurements were so
valuable at Jupiter that they funded and are flying the ~$1B Juno mission,
which will do similar science there.

At one time, funding for Cassini’s full extended mission to the end of
2017 had been approved. Then NASA’s
planetary budget was cut and now the mission has to seek approval again to
continue operating.

I suspect that the Cassini team is scrubbing its budgets to look for
ways to reduce costs and still enable the close up orbits at the end of the
mission. That team has stated before,
though, at a large portion of the budget goes to simply operating the
spacecraft safely. Cuts would have to
come from reducing science operations. (I
don’t know if the orbital ballet enabling the close-in orbits would allow them
to be done earlier to shorten the length of the extended mission.)

However, the real solution is
for NASA to receive more funding so that it doesn’t have to make choices that
turn off a highly productive mission to save 4% of its planetary science budget.

If you would like to see Cassini make those final orbits, skimming the
rings and Saturn’s atmosphere, I encourage you to join
the Planetary Society’s campaign to have NASA’s budgeting program receive
more funding.

Appendix: Budgets for extended missions

For those who like to see the numbers, I’ve gone through NASA’s Fiscal
Year 2014 budget proposal to pull out the planned budget for extended missions
and the budget for Curiosity’s last year of funding in its prime mission.

The budget doesn’t break out funding for the Cassini or Lunar
Reconnaissance orbiter. Public accounts
have put the Cassini extended mission operations at $60M to $65M a year. The MESSENGER orbiter at Mercury is expected
to run out of fuel before the start of FY15, and will not be needing funds
beyond FY14.

All extended missions currently are funded through the end of
FY14. The crunch begins in FY15 when the
budget numbers show a funding cut and Curiosity needs new funding to begin its
extended mission.

FY14

FY15

Cassini

$50.0

Saturn

Curiosity

47.1*

Mars

Mars extended missions

$82.3

Mars

Mars Reconnaissance
Orbiter

$30.5

Mars

Opportunity

$14.7

Mars

Mars Odyssey

$12.8

Mars

Messenger

$4.9

Mercury

Mars Express

$2.2

Mars

Lunar Reconnaisance
Orbiter

?

?

Moon

Extended Missions

$115.1

$82.3

* Prime mission funding

Funding identified in NASA FY14 budget
proposal for extended missions

While NASA’s budget documents don’t show spending categories within the Outer Planets budget category, the chart
below shows sharp cuts to this account.
By FY16, funding is too low to support the Cassini mission (and may represent
NASA’s contribution to Europe’s JUICE Jupiter/Ganymede mission). There
may be some funding for Cassini in FY15.
Proposed budgets following FY16 show approximately flat funding.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.